Transmitter-receiver device and a communication system

ABSTRACT

The invention concerns a transmitter-receiver device (A, B) which comprises a receiver unit (RXA) for receiving optical signals and transmitter unit (TXA) for transmitting optical signals. Furthermore, the transmitter-receiver device (A, B) comprises a supervising unit (CUA) which supervises the functions of the receiver unit (RXA) and the transmitter unit (TXA). Furthermore, the transmitter-receiver device (A, B) comprises a transmitter circuit which transmits optical communication signals in response to a balanced electric input signal. The invention also concerns a communication system comprising two transmitter-receiver devices (A, B). Through the structure of the invention is by relatively simple means a well functioning device achieved, which, inter alia, makes it possible to supervise the status of the two transmitter-receiver devices (A, B) in an advantageous manner.

BACKGROUND OF THE INVENTION AND PRIOR ART

The present invention concerns a transmitter-receiver device which maybe used to transfer information optically. Such a transmitter-receiverdevice may form part of a communication system for bi-directionaltransfer of optical signals. The invention also concerns such acommunication system.

FIG. 1 shows schematically an example of a bi-directional communicationsystem according to the prior art for transferring optical signals. Thesystem comprises a first transmitter-receiver device A with a receiverunit RXA and a transmitter unit TXA. The transmitter-receiver device Acommunicates with a similar transmitter receiver device B. Also thetransmitter-receiver device B thus comprises a transmitter unit TXB anda receiver unit RXB. The transmitter unit TXB transmits optical signalsover a first optical fibre F1 to the receiver unit RXA. In a similarmanner, the transmitter unit TXA transmits optical signals over a secondoptical fibre F2 to the receiver unit RXB. In such a system, the lightthat is transmitted from the respective transmitter unit TXA, TXB oftenhas an essentially constant average power, i.e. light is normallytransmitted all the time. The information transfer is carried outthrough a suitable modulation of this light signal.

The receiver units RXA, RXB may have an output, UA, UB, respectively,which for example may assume two logical values depending on if theoptical power received in the receiver unit RXA, RXB exceeds a certainvalue. This output UA, UB may for example be connected to an indicatorIA, IB, for example in the form of a light emitting diode. Such anindicator IA, IB may for example emit light if the respective receiverunit RXA, RXB receives light. In such a manner, the respective indicatorIA, IB may indicate that the connection over the fibre F1, F2 works. Thesignal from the output UA or UB may also be connected to a networkmanagement system NMS. Such a network management system NMS supervisesthe communication system and makes it possible to, from a completelydifferent position than where the transmitter-receiver devices A and Bare located, supervise whether the bi-directional communication systemworks. The lines 131, 133, 135, and 137 are intended to transferinformation carrying signals, for example as electric signals, to andfrom the transmitter and receiver units RXA, TXA, TXB, and RXB.

In the system according to FIG. 1, for example the transmitter-receiverdevice B may be arranged in a home and the second transmitter-receiverdevice A may constitute a centrally located device which transmitssignals to the home and receives signals from the home. In order tosupervise the system it is desirable to connect the device B to thenetwork management system NMS. If, for example, the device B ispositioned in a home or in an office, this device often has no otherconnection out from the home or the office than via the optical fibresF1 and F2. It is actually conceivable to connect the device B to anetwork management system via the fibre F1. However, it is relativelyexpensive and complicated to, in addition to the normal informationsignals, also transmit signals concerning the network management on thesame fibre F1. Furthermore, this network management does not work incase of a breakage of the fibre F1. Since the network management systemNMS is intended to supervise the function of the network, it isdesirable that this supervision works also in case an error occurs inthe communication between the devices A and B. It is, of course,conceivable that the network management system NMS may supervise thedevice B via another line than via F1 or F2, for example in the homethere may be a telephone modem, over which this supervision takes place.This constitutes, however, a complicated solution for transferringsignals to the network management system NMS. It is, of course, alsopossible that a person goes to the device B in order to personally checkif, for example, the indicator IB is lit. This may possibly beacceptable within, for example, an office where the distance to B may beshort. However, it becomes much more complicated to send a person to thedevice B if this device is located at a long distance from the positionwhere the person normally is.

As a background to the present invention, also so-called eye-safe fibrecommunication systems should be mentioned. A problem with fibrecommunication systems is that the light intensity which is transmittedover the fibres may be relatively high. If, for example, a fibre isbroken and if somebody looks into the fibre, damages of the eye mightoccur. If a fibre is broken or damaged, it is therefore desirable to cutoff the light signal which is transmitted over this fibre. For exampleU.S. Pat. No. 5,136,410 describes such a system.

With reference to FIG. 1 it will now briefly be described how aneye-safe system may work. Suppose that a breakage takes place of thefibre F1. The output UA thereby indicates that no light is received inthe receiver unit RXA. A supervising unit (not shown in FIG. 1) may thencut off the transmission of light from the transmitter unit TXA. At thereceiver unit RXB it is thereby detected that no light is received.Another supervising unit in connection to the device B therebyimmediately cuts off the transmission of light from the transmitter unitTXB. Thereby, no harmful light exists on the broken fibre F1. In orderto check if the connection via the fibre F1 works again, the respectivetransmitter unit TXA, TXB often transmits short light pulses at regularintervals. These light pulses are so short that they are not harmful tothe eye. When one receiver unit RXB receives such a light pulse, asimilar light pulse is immediately transmitted by the transmitter unitTXB. As long as the breakage of the fibre F1 is the case, the receiverunit RXA does not detect any such light pulse. If, however, the fibre F1works again, the receiver unit RXA will detect such a response pulsefrom TXB immediately after TXA having transmitted a pulse to RXB. Theconnection thus works again and the respective transmitter unit TXA, TXBmay now continuously transmit light. The time between the light pulsesin such a safety system is usually quite long, for example U.S. Pat. No.5,136,410 mentions that the time between these pulses is about 49seconds.

The line 133 (and 135) shown in FIG. 1 may constitute a pair ofconductors on which a balanced electric signal is present.

The transmitter unit TXA (and TXB) therefore normally has a transmittercircuit comprising a light source and arranged to operate said lightsource to transmit optical communication signals in response to electricinput signals from a first and second circuit point between whichcircuit points a balanced electric input signal is intended to bepresent.

Different transmitter circuits of the above mentioned kind are known. Apair of electric conductors has a certain characteristic impedance forexample 100 ohm. In order to avoid undesired reflections, such a pair ofelectric conductors should in its end point be connected to a load whichcorresponds to the characteristic impedance.

It should be noted that by a balanced signal is meant that the signalthat is present on the pair of conductors is such that the voltages oncorresponding points on the two conductors are of the same magnitude buthave opposite polarity to a reference potential. This referencepotential is usually earth potential. With an unbalanced signal (or“single-ended”) is meant that the signal, i.e. the voltage variation, isonly present on one conductor, while the other conductor, or referencepotential, is at a constant potential, usually on earth potential.

On a pair of conductors with a balanced signal, due to noise or otherphenomena, a signal which is superposed on the two conductors may occur,a so-called common mode signal, which signal may vary with time. Thissignal is often undesired and should therefore be suppressed. This isoften done with the help of, for example, transformers, baluns (a balunis a device which converts a balanced signal to an unbalanced signal)and differential amplifiers.

Also when a balanced electric signal is to be converted to an opticalsignal, such an undesired superposed signal need to be suppressed inorder for the light source, which transmits the optical signal, to becorrectly operated. According to the prior art, this has usually beendone by first converting the balanced electric signal to an unbalancedelectric signal.

FIG. 2 shows an example of the prior art. The electric balanced inputsignal is here present on a twisted pair 30. The balanced signal isconverted to an unbalanced signal with the help of a balun 41 and atransformer 42. The circuit also comprises a termination resistance 43which is adapted to the characteristic impedance of the twisted pair 30.Thereafter follows one or more circuits 44, which i.a. produce asuitable bias current and a modulation current, wherein the totalcurrent drives the light source 20.

Also EP-A-0 542 480 shows an example of a transmitter circuit. Thetransmitter circuit comprises two differentiators and an amplifier fordriving a light emitting diode.

The prior known solutions are relative complicated and expensive, sincethey often comprise relatively complicated and expensive components,such as active components or transformers. Furthermore, knowntransmitter circuits often have a relatively high current consumption.

It should be noted that by active components is meant components whichproduce a gain or a switching, for example transistors, integratedcircuits, and diodes.

SUMMARY OF THE INVENTION

An object of the present invention is to achieve a transmitter-receiverdevice which may be implemented with relatively simple means and whichenables a reliable communication by means of optical signals. An objectthereby is to achieve a transmitter-receiver device which may be used ina bi-directional optical communication system with improved networkmanagement possibilities compared to previous systems and whichcomprises a transmitter circuits which functions well and which is moresimple than typical known transmitter circuits. The above-describeddisadvantages with such previous systems should therefore be avoidedwith the present invention.

These objects are achieved with a transmitter-receiver device accordingto claim 1. Since the transmitter-receiver device is arranged with sucha third output and a status signal which indicates if thetransmitter-receiver device is in the test mode or not, thetransmitter-receiver device may be used in a bi-directional system wherea network management system connected to one side may also supervise thestatus of the other side. This will become clear from the descriptionbelow. With the invention, also the advantage is achieved that thebalanced signal to the transmitter unit does not need to be converted toan unbalanced signal. The transmitter circuit can thereby be realisedwith simple and inexpensive components.

It should be noted that by “normal operation conditions” is meant thatthe transmitter circuit works within voltages and currents which arenormal for the transmitter circuit, where, as has been mentioned, alsoan undesired superposed voltage may be present on the balanced electricsignal. However, for example extreme voltage peaks may be considered toconstitute non-normal operation conditions.

An embodiment of the invention is clear from claim 2. Since the timebetween the light pulses is so short, the device according to theinvention may suitably be used in a system with a safety function whichoperates essentially quicker than according to the above-describedsystem. Furthermore, this short time makes it possible that the statusof different signals of one side of a bi-directional system correspondsto the status signals on the other side of the system. As also willbecome clear from the description below, such a short time also makes itpossible to in a simple manner measure with a normal optical power meterwhether a connection is the case between two parts in a bi-directionalfibre optic system.

Still another embodiment is clear from claim 3. According to thisembodiment, it may, via the fifth output, be supervised whether thereceiver unit receives an information carrying signal over a workingoptical conduction path.

Another embodiment is clear from claim 4. With the help of the seventhoutput, it may be supervised whether the transmitter unit transmitsinformation over a working connection.

Another preferred embodiment is clear from claim 5. This mirroredsymmetry may preferably be achieved if the first and the second circuitbranches comprise components with exactly the same value oncorresponding positions in the respective circuit branch. The featurethat the electric properties of the components correspond to each othermeans however that it does not have to be exactly the same components onthe two circuit branches, as long as the electric properties of the twocircuit branches are the same. For example, the electric propertieswhich together are the case in the component or components which arearranged between two nodes in one of the circuit branches ought tocorrespond to the same electric properties which together are the casein the component or components which are arranged between thecorresponding two nodes in the second circuit branch.

Since the transmitter circuit is formed with this symmetry, it ispossible to, with simple components, maintain a balanced signal all theway to the light source. Furthermore, it is achieved that the lightsource is only modulated by the voltage difference between the abovementioned first and second circuit points. The current through the lightsource is thus independent of a possible common-mode signal which ispresent on said first and second circuit points.

A further embodiment is clear from claim 6. Thereby, a suitablebias-current through the light source may be obtained in a simplemanner. Suitably at least one of said first and second constant voltagesmay be adjustable. Hereby, the bias-current may simply be adjustedwithout influencing the modulation current.

A further embodiment is clear from claim 7. Hereby, the advantages ofthe transmitter circuit are achieved in a simple manner and withinexpensive components. Preferably, no transformers or magneticcomponents are used in the transmitter circuit. As has been mentionedabove, also no balun is used.

The receiver unit of the transmitter-receiver device suitably has anamplifier circuit, which amplifies the incoming signal to a suitablelevel.

In this context prior known amplifier circuits should be mentioned. U.S.Pat. No. 5,917,639 and U.S. Pat. No. 6,055,094 describe different kindsof known amplifier circuits.

An amplifier circuit may for example be used when it is desired that asignal should lie at a predetermined level. Such an amplifier circuit isoften called AGC (Automatic Gain Control).

A disadvantage with prior known amplifier circuits is that it is oftendifficult to control the amplification for different kinds of signals.For example, some signals may comprise pulses of a very high frequencyand other signals may comprise pulses with relatively long pausesbetween the pulses. When pulses arrive with long pauses between thepulses, the amplifier circuit may tend to amplify the signal, which maymean that when then pulses of a high frequency arrive, the amplificationmay be too high, which may lead to different problems, the amplifier mayfor example be saturated and the amplification may become non-linear.

A preferred embodiment of the present invention is clear from claim 8.Since the amplification is set in accordance with the control signalwhich gives the lowest amplification, the risk is reduced that a toohigh amplification is set when the control signals from the first andthe second control unit differ.

Another preferred embodiment of the invention is clear from claim 9.Hereby is in a simple manner achieved that the amplifier circuit takesdifferent kinds of signals into account and ensures that a too highamplification, which could have been caused by some of the signals, isavoided.

A further embodiment is clear from claim 10. Through this embodiment theproblem is avoided that a signal with relatively long pauses between thepulses may lead to a too high amplification This is avoided since thesecond control unit is arranged to sense this kind of signal.

The receiver unit of the transmitter-receiver device suitably has anoptical input stage. Such an input stage may be followed by furtheramplifiers.

In this context prior known optical input stages should be mentioned.

FIG. 3 shows schematically an example of such an optical input stageaccording to the prior art. The input stage comprises a light sensitivemember 301. The light sensitive member 301 may for example constitute aphoto-diode. The light sensitive member 301 delivers an electric signalin response to an optical input signal, for example from an opticalfibre (not shown in the figure). According to the shown example, thecathode of the photo-diode 301 is connected to a bias voltage V1. Thecircuit comprises an amplifier component 302, which is often calledpreamplifier. The amplifier component 302 has a first input 304 whichreceives an electric signal from the light sensitive member 301. Theamplifier component 302 influences the amplification of the electricsignal and delivers an amplified output signal via a first output 306.The shown photo-diode 301 delivers a current into the input 304, whereinthe strength of the current depends on detected light. The amplifiercomponent 302 converts the current to a voltage at the output 306. Thetransfer function therefore gets the unit V/A, i.e. ohm. Theamplification of the amplifier component 302 may thus be stated in ohm.The amplifier component 302 may also comprise an internal amplificationcontrol unit 308 which for example may be arranged to reduce theamplification at too high currents.

The English abstract of JP-A-10284955 shows an example of this kind ofoptical input stage. This document shows such an input stage with acontrol circuit for controlling the amplification in response to anaverage value of the optical input power.

Also WO99/28768 shows an optical input stage where a control circuitcontrols a variable impedance element in the form of a diode connectedto an amplifier input.

The English abstract of JP-A-09298426 shows an optical input stage witha preamplifier. In this case the preamplifier has a special input wherea control signal may be connected for controlling the amplification ofthe preamplifier. However, an optical input stage usually lacks aspecial input for controlling the amplification. An example of an inputstage is the one which is sold with the name MC2006 of the fabricationMicrocosm. For example this input stage comprises an internal controlunit for reducing the amplification at too high currents. However, theinput stage lacks a special input for being able to control theamplification.

A further preferred embodiment of the present invention is clear fromclaim 11. According to this embodiment it is possible to control theamplification in an optical input stage with an amplifier componentwhich does not have any special input intended for controlling theamplification.

The filter unit may for example constitute a capacitor. This filter unitdisconnects a possible direct current from the light sensitive member.Instead the control unit is connected to the first input. The power ofthe input signal at the first input is thus controlled with the help ofthe control unit instead of with the help of a direct current from thelight sensitive member. Thereby, the power of the output signal from thesecond output may be influenced with the help of the control unit, i.e.the purpose to be able to control the amplification of the amplifiercomponent is achieved.

Another embodiment of the invention is clear from claim 12. According tothis embodiment, a light sensitive member may thus be used where thecurrent into the first input depends on detected light intensity.

Still an embodiment is clear from claim 13. According to thisembodiment, for example a photo-diode of the kind which has beendescribed above may thus be used as light sensitive member.

Still an embodiment is clear from claim 14. Hereby is prevented that theamplifier component is set at a too high amplification.

A further embodiment is clear from claim 15. For example if the circuitis arranged such that always a certain current is input via said firstinput, then the second diode unit may be arranged for preventing acurrent in the opposite direction.

As has been mentioned above, a further object of the invention is toachieve a communication system. The object thereby is to achieve asystem with improved network management possibilities compared toprevious systems.

This object is achieved with a communication system according to claim16.

With such a communication system, the above-described advantages areachieved. Such a system makes it possible, by supervising the status ofone side of the system, to also have information about the status of thecorresponding signals of the other side of the system.

A preferred embodiment of the communication system is clear from claim17. With the help of the network management system, the function of bothtransmitter-receiver devices may thereby be supervised even if thenetwork management system is only connected to one of the devices.

Another preferred embodiment of the communication system is clear fromclaim 18. The network management system may hereby supervise whether aworking connection is the case between the two transmitter-receiverdevices.

Another preferred embodiment of the communication system is clear fromclaim 19. The network management system may hereby supervise the statusof several of said outputs.

Still an embodiment of the communication system is clear from claim 20.By supervising one of the transmitter-receiver devices, information mayhereby also be obtained concerning the corresponding outputs of thesecond transmitter-receiver device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically a bi-directional fibre optic communicationsystem according to the prior art.

FIG. 2 shows a transmitter circuit according to the prior art.

FIG. 3 shows schematically an optical input stage according to the priorart.

FIG. 4 shows schematically a transmitter-receiver device according tothe present invention.

FIG. 5 shows the course of events in time for the function of theinvention when a disruption occurs on an optical connection.

FIG. 6 shows a similar course of events in time as FIG. 5 when aconnection is created again in the previously disrupted opticalconnection.

FIG. 7 shows a transmitter circuit which may form part of an embodimentof the invention.

FIG. 8 shows an equivalent circuit of the transmitter circuit accordingto FIG. 7.

FIG. 9 shows another embodiment of the transmitter circuit.

FIG. 10 shows the principle of preferred embodiments of the transmittercircuit.

FIG. 11 shows schematically a first embodiment of an amplifier circuitwhich may form part of the invention.

FIG. 12 shows a second embodiment of the amplifier circuit.

FIG. 13 shows a third embodiment of the amplifier circuit.

FIG. 14 shows a fourth embodiment of the amplifier circuit.

FIG. 15 shows schematically a simple embodiment of a circuit which mayform part of the invention.

FIG. 16 shows another embodiment of the circuit.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Examples of different embodiments of the invention will now bedescribed. First a transmitter-receiver device and a communicationsystem will be described. Then different parts which may form part ofthe transmitter-receiver device will be described.

FIG. 4 shows a transmitter-receiver device A according to an embodimentof the present invention. In a communication system according to theinvention, suitably two such transmitter-receiver devices A, B formpart, which devices are connected in the manner shown in FIG. 1.

FIG. 4 shows a first optical conduction path F1. Furthermore, FIG. 4shows a second optical conduction path F2. These first and secondoptical conduction paths F1, F2 may suitably consist of optical fibres.The transmitter-receiver device A comprises a receiver unit RXA whichreceives light from the optical fibre F1. Furthermore, there is atransmitter unit TXA which transmits light on the optical fibre F2. Thereceiver unit RXA has a first output 101 which indicates whether thereceiver unit RXA receives light. The transmitter unit TXA has a firstinput 103 which controls whether the transmitter unit TXA shall transmitlight in response to an electric input signal.

The device also comprises a supervising unit CUA. The supervising unitCUA has a second input 105 connected to the first output 101 and asecond output 107 connected to the first input 103. The supervising unitCUA is arranged to prevent the transmitter unit TXA from continuouslytransmitting light when the supervising unit CUA detects that thereceiver unit RXA does not receive light. Furthermore, the supervisingunit CUA is arranged to change to a test mode when it detects that thereceiver unit RXA does not receive light. During the test mode, thesupervising unit CUA controls the transmitter unit TXA to intermittentlytransmit short light pulses on the second optical fibre F2. Thisfunction is thus similar to previously known eye-safe systems.

The supervising unit CUA has a third output 109 where a status signalindicates whether the device is in said test mode. The supervising unitCUA may be implemented in hardware or in software. For example, thesupervising unit CUA may consist of a so-called microcontroller.

The third output 109 may be connected to a first indicator 119 and/or toa network management system NMS. If the third output 109 is connected toan indicator 119, this indicator 119 may, according to a preferredembodiment, emit light if the device is in said test mode and be put outif the device is not in the test mode. The indicator 119 may, forexample, consist of a red light emitting diode.

As has been described above, said test mode means that the transmitterunit TXA only intermittently transmits short light pulses. If the deviceA forms part of a bi-directional system with a corresponding device B,this means that the receiver unit TXB does not detect any continuouslight on the fibre F2 when the device A has entered into said test mode.This means that also the device B enters into the test mode, wherein acorresponding output 109 in the device B has the same status as theoutput 109 in the device A. According to a preferred embodiment of theinvention, the distance between the light pulses in the test mode isless than 1 second, preferably less than 0.1 second and most preferredless than 5 milliseconds. The length of the pulses in the test mode issuitably 0.1%–25%, preferably 3%–20%, and most preferred 5%–13% of thedistance between light pulses.

With reference to FIGS. 5 and 6 it will now be described how the deviceis arranged to function when a disruption occurs in the connectionbetween a first and a second device A, B. Suppose that a disruptionoccurs in the fibre F1 at the time T1. The upper graph 151 in FIG. 5shows the light intensity which reaches RXA. This intensity thus sinksat the time T1 when a disruption takes place in the fibre F1. Thesupervising unit CUA thereby controls the device A such that it changesinto said test mode. The second graph 152 in FIG. 5 shows the light thatis transmitted from TXA. Shortly after the time T1, because of a smalldelay in the electronics, the device A changes to said test mode.According to the shown embodiment, TXA transmits, during the test mode,intermittently short light pulses, wherein each light pulse is only 100μs. The distance between the light pulses is, according to thisembodiment, 900 μs. The light pulses which are transmitted from TXA arereceived by RXB. Because of the transfer time in the fibre F2, thesepulses are received somewhat later than when they are transmitted. TXBis controlled by a supervising unit in such a manner that TXB transmitslight as soon as RXB receives light. The graph 153 in FIG. 5 shows thelight which is transmitted from TXB. As can be seen in FIG. 5, TXBtherefore transmits light pulses in the same manner as TXA during thetest mode but with a small time delay caused by a delay in the transferand in the electronics. This means that if the device A changes into thetest mode, then also the device B changes into this mode.

FIG. 6 shows the same graphs as FIG. 5 when the connection along thefibre F1 is re-established. It is assumed that this connection isre-established at the time T2. When the next pulse is transmitted fromTXA, RXA receives an answer from TXB before TXA has been switched off.This means that TXA is allowed to continue to be switched on, whichmeans that RXB senses a continuous input power and therefore lets TXB beswitched on. The connection is thus re-established.

Since the device B is in the test mode when and only when the device Ais in said test mode (except for a small time delay), it follows thatsaid third output 109 of the device A always has the same status as thecorresponding output of the device B. By supervising the status of thisoutput 109 in the device A, information is thus obtained also concerningthe status of the corresponding output in the device B. This means thata network management system NMS connected to the device A indirectlyalso supervises the device B.

There are further advantages in that the device has such a short timedistance between the pulses when it is in said test mode. One reason isthat a user who, for example, connects and disconnects optical contactsimmediately will see if the connection works or not. Since the test modeworks with said short time distance, information may immediately beobtained of whether the optic connection works. Another advantage isthat one sometimes with an optical power meter would like to measure ifthere is power in a fibre or not. This is usually done in that the fibreis disconnected from a receiver unit and connected to a power meter.Since the fibre is disconnected, the device changes to the test mode.Since the pulses during the test mode arrive with such a short timedistance, a normal optical power meter will measure a certain power inthe fibre. This measured power corresponds to the average power in thefibre. This average power during the test mode is, for example, tentimes lower than the normal power when the connection is not in the testmode. With the help of the power meter, information may thus immediatelybe obtained concerning whether test pulses are received at the powermeter. This means that the fibre in question is not broken, sinceotherwise no test pulses would reach the power meter. With aconventional slower device, on the other hand, where the distancebetween the pulses is essentially longer, one would with a power meterfor the most part measure no optical power at all and sometimes a littleoptical power. This makes fault-tracing essentially more difficult in asystem with several fibres, since it may thereby be difficult todetermine in which fibre there is a disruption.

A further advantage of the invention is that the device, in spite of thefact that it works so fast, may protect against eye damages in a similarmanner as previous slower working systems.

FIG. 4 also shows that the transmitter-receiver device A may havefurther components. FIG. 4 thus shows that RXA has an output 141 wherean information carrying signal from RXA is transmitted. In a similarmanner, TXA has a further input 143 where an information carryingelectric signal is received by TXA. The receiver unit RXA also has afourth output 111 which is connected to the supervising unit CUA. At thefourth output 111, a signal is the case which indicates whether thereceiver unit RXA receives an information carrying signal via the firstoptical fibre F1. Furthermore, the supervising unit CUA has a fifthoutput 113. On this fifth output 113, a status signal is the case whichdepends on the status of the signal of the third output 109 and thestatus of the signal from the fourth output 111. The fifth output 113may suitably be connected to a second visual indicator 121 and/or to thenetwork management system NMS. The supervising unit CUA may suitably bearranged such that the second indicator 121 is lit if the fourth output111 indicates that the receiver unit RXA receives an informationcarrying signal at the same time as the output 109 indicates that theconnection works. The reason that the output 111 is connected to thesupervising unit CUA instead of to be directly connected to, forexample, the second indicator 121 is that when the optical connectiondoes not work, some light may sometimes still be received which wouldmean that the second indicator 121 would twinkle.

The transmitter unit TXA also has a sixth output 115 which is connectedto the supervising unit CUA. At the sixth output 115 a signal is thecase which indicates if the transmitter unit TXA receives an electricinformation carrying signal on the input 143. The supervising unit CUAalso has a seventh output 117. The seventh output 117 has a statussignal which depends on the status of the signal of the third output 109and the status of the signal from the sixth output 115. This seventhoutput 117 may be connected to a third indicator 123 and/or to thenetwork management system NMS. The supervising unit CUA is suitablyarranged such that-said seventh output 117 has a certain status if boththe output 109 shows that the connection works and the output 115 showsthat TXA receives an information carrying signal.

The supervising unit CUA is suitably arranged such that the status ofthe signals at said third, fourth, fifth, sixth, and seventh outputs(109, 111, 113, 115, 117) fulfills the following status schedule:

Fourth output Sixth output Third output = 0 Third output = 1 0 0 Fifthoutput = 0 Fifth output = 0 Seventh output = 0 Seventh output = 0 0 1Fifth output = 0 Fifth output = 0 Seventh output = 0 Seventh output = 11 1 Fifth output = 0 Fifth output = 1 Seventh output = 0 Seventh output= 1 1 0 Fifth output = 0 Fifth output = 1 Seventh output = 0 Seventhoutput = 0wherein the first column indicates the status of the fourth output 111,the second column indicates the status of the sixth output 115, in thethird column the third output 109 has status=0 and in the fourth columnthe third output 109 has status=1, and wherein the respective statusstands for the following:

-   Third output 109=1, the connection works and the    transmitter-receiver device is not in said test mode;-   Third output 109=0, the transmitter-receiver device is in said test    mode;-   Fourth output 111=1, the receiver unit RXA receives an information    carrying signal;-   Fourth output 111=0, the receiver unit RXA does not receive an    information carrying signal;-   Fifth output 113=1, indicates that there is a working optical    connection with an information carrying signal to the receiver unit    RXA;-   Fifth output 113=0, indicates that there is no working optical    connection with an information carrying signal to the receiver unit    RXA;-   Sixth output 115=1, the transmitter unit TXA receives an electric    information carrying input signal;-   Sixth output 115=0, the transmitter unit TXA does not receive an    electric information carrying input signal;-   Seventh output 117=1, indicates that there is a working optical    connection with an information carrying signal which is transmitted    from the transmitter unit TXA;-   Seventh output 117=0, indicates that there is no working optical    connection with an information carrying signal which is transmitted    from the transmitter unit TXA.

As has been mentioned above, the device A may be used in a communicationsystem together with a corresponding device B. When these devices A, Bform part of a communication system of the kind that is shown in FIG. 1,the great advantage is obtained by the invention that the status of saidthird 109, fifth 113, and seventh 117 outputs of onetransmitter-receiver device A is exactly the same as the status of thecorresponding outputs in the device B. This means, as has been explainedabove, that the status of the third output 109, which indicates whetherthe connection works, is the same both in the device A and the device B.Furthermore, the status of the fifth output 113, which indicates whetherthe device A receives an information carrying signal, is the same as thestatus of the seventh output 117 of the device B, which seventh outputof the device B indicates that TXB transmits an information carryingsignal. In a corresponding manner, the status of the fifth output 113 ofthe device B is the same as the status of the seventh output 117 of thedevice A. By only supervising, for example, the device A, one knows, forinstance, that if the seventh output 117 has a certain status, then aninformation carrying signal is transmitted on the optical fibre F2 fromthe transmitter unit TXA, but, furthermore, one knows that this signalis received by the receiver unit TXB, since the seventh output alsoindicates that the optical connection over the fibres F1 and F2 betweenthe device A and the device B works.

A transmitter-receiver device according to the invention may suitable bearranged on a circuit card. Such a device A may, for example, bearranged in or in connection to a wall in a home or in an office. Thedevice may, of course, also form part of a centrally located devicewhich is controlled by a network operator which transmits and receivessignals to a device arranged in a home or in an office.

It should be noted that by “light” is in this application notnecessarily meant that the light must be visible. Also invisibleelectromagnetic radiation may be transferred over the optical conductionpaths.

Now preferred embodiments of the transmitter circuit which forms part ofthe invention will be described.

FIG. 7 shows a transmitter circuit arranged between an electric line andan optical fibre. The figure shows a twisted pair 30 of conductors 31,32. These conductors are connected to a first 11 and a second 12 pointof the transmitter circuit. It should be noted that other kinds ofconductors than a twisted pair 30 are possible. For example, a ribboncable is thus conceivable or simply two conductors on a circuit card. Abalanced electric input signal is conducted to the first 11 and second12 points. The transmitter circuit converts this signal to an opticalsignal which is transmitted from a light source 20. An optical conductor35 can conduct light from the light source 20.

The transmitter circuit has a first circuit branch 21 and a secondcircuit branch 22. The first circuit branch 21 extends from the firstpoint 11 via a third point 13 to a fourth point 14. The second circuitbranch 22 extends from the second point 12 via a fifth point 15 to asixth point 16. The light source is connected between the third point 13and the fifth point 15. The first circuit branch 21 comprises a firstcapacitor C₁ and a first resistance R_(M1) which are connected in seriesafter each other between the first point 11 and the third point 13. In acorresponding manner, the second circuit branch 22 comprises a secondcapacitor C₂ and a second resistance R_(M2) which are connected inseries between this second point 12 and the fifth point 15.

Furthermore, the first circuit branch 21 comprises a third resistanceR_(B1) which is arranged between the third point 13 and fourth point 14.The fourth point 14 is arranged to be at a first constant voltage V_(A).In the shown example, this first voltage V_(A) is earth potential.Furthermore, the second branch 22 comprises a fourth resistance R_(B2)which is arranged between the fifth point 15 and the sixth point 16. Thetransmitter circuit is arranged such that a second constant voltageV_(B) is the case at the sixth point 16. One of said first V_(A) andsecond V_(B) constant voltages may suitably be adjustable. For example,the second constant voltage V_(B) may be adjustable. Thereby, thebias-voltage through the light source 20 may be simply adjusted withoutinfluencing the modulation current.

The transmitter circuit also comprises a third circuit branch 23. Thisthird circuit branch 23 extends from a point 17 on the first circuitbranch 21 to a point 18 on the second circuit branch 22. On the thirdcircuit branch 23, a termination resistance R_(T) is arranged. By asuitable choice of this termination resistance R_(T), the impedance ofthe circuit may be adapted to the characteristic impedance of theconduction pair 30 which is connected to the transmitter circuit. Thecomponents which are positioned on the first 21 and second 22 circuitbranches are chosen such that the transmitter circuit is formed with asymmetry. The symmetry is such that a balanced drive voltage is the casebetween the third 13 and the fifth 15 points. The balanced drive voltageis independent of a possible superposed voltage which is present on theinput signal, i.e. on the two first 11 and second 12 points. In thismanner, the light source 20 is modulated exactly in response to thevoltage difference between the two conductors 31, 32 which are connectedto the first 11 and second 12 points, respectively.

The easiest manner of achieving said symmetry is that the electricproperties of the components which are arranged between different nodeson the first circuit branch 21 correspond to the same electricproperties of the components which are arranged in correspondingpositions in the second circuit branch 22. This purpose may simply beachieved if the first capacitor C₁ has the same value as the secondcapacitor C₂, the first resistance R_(M1) has the same value as thesecond resistance R_(M2), and the third resistance R_(B1) has the samevalue as the fourth resistance R_(B2).

An advantage with the invention is that all components which arearranged on the respective circuit branch 21, 22 between the first 11and the fourth point 14 and between the second 12 and the sixth point16, respectively, may be passive components. In the shown case, thesecomponents consist only of capacitors and resistances. Hereby, also theuse of transformers or more expensive magnetic components is avoided.

A suitable bias-current through the light source 20 is selected by thechoice of the second constant voltage V_(B), the third resistanceR_(B1), and the fourth resistance R_(B2). The scaling factor between thevoltage of the balanced input signal and the modulation current throughthe light source 20 is selected by a suitable choice of the firstresistance R_(M1) and the second resistance R_(M2). The first C₁ and thesecond C₂ capacitors prevent a superposed voltage from reaching thelight source 20 in the form of a direct current.

In order to show that the current through the light source 20 isindependent of a possible superposed voltage on the conduction pair 31,32, reference is made to FIG. 8. FIG. 8 shows an equivalent circuit ofthe transmitter circuit according to FIG. 7. As a light source 20, forexample a light emitting diode or a laser diode may be used. A simplemodel of such a light source 20 is an independent voltage source V_(S)in series with a resistance R_(S). Z_(M1) corresponds to the firstcapacitor C₁ in series with the first resistance R_(M1). In acorresponding manner, Z_(M2) corresponds to the second capacitor C₂ inseries with the second resistance R_(M2). In FIG. 2 also the currentsI₁, I₂, and I_(S) as well as the voltages V₁, V₂, U₁, and U₂ are marked.

With reference to FIG. 8 the following equations may be formed.

$\begin{matrix}{I_{1} = \frac{V_{1} - U_{1}}{Z_{M\; 1}}} & (1) \\{U_{2} = {U_{1} + V_{S} + {I_{S} \cdot R_{S}}}} & (2) \\{I_{2} = \frac{V_{2} - U_{2}}{Z_{M\; 2}}} & (3) \\{I_{S} = {\frac{V_{B} - U_{2}}{R_{B2}} + I_{2}}} & (4) \\{I_{S} = {\frac{U_{1}}{R_{B\; 1}} - I_{1}}} & (5)\end{matrix}$

Since the transmitter circuit is symmetrically formed, also thefollowing equalities are fulfilled.R_(B)=R_(B1)=R_(B2)  (6)Z_(M)=Z_(M1)=Z_(M2)  (7)

With the help of (1) to (7), the following expression may be derived.

$\begin{matrix}{I_{S} = \frac{{\left( {V_{2} - V_{1}} \right)R_{B}} - {V_{S}\left( {R_{B} + Z_{M}} \right)} + {V_{B}Z_{M}}}{{R_{B}\left( {{2Z_{M}} + R_{S}} \right)} + {Z_{M}R_{S}}}} & (8)\end{matrix}$

From (8) is clear that the current through the light source only dependson the difference between V₂ and V₁. If, for example, both V₂ and V₁suddenly increase, for example with 100 V, the current through the lightsource is not influenced.

In order to determine the bias-current, V₂ and V₁ may be set to be equal(V₂=V₁). Thereby, the following is derived.

$\begin{matrix}{I_{S\; B} = \frac{{V_{S}\left( {R_{B} + Z_{M}} \right)} + {V_{B}Z_{M}}}{{R_{B}\left( {{2Z_{M}} + R_{S}} \right)} + {Z_{M}R_{S}}}} & (9)\end{matrix}$

If it is assumed that Z_(M) is a resistance in series with a capacitor,as in FIG. 7, then Z_(M) goes towards infinity at the frequency 0 Hz.Thereby, the following is obtained when Z_(M) goes towards infinity.

$\begin{matrix}{I_{S\; B} = \frac{V_{B} - V_{S}}{{2R_{B}} + R_{S}}} & (10)\end{matrix}$

The expression (10) thus shows the direct current (the bias-current)through the light source. The modulation current is the total current(8) minus the bias-current (9). The modulation current is thus:

$\begin{matrix}{I_{SM} = \frac{\left( {V_{2} - V_{1}} \right)R_{B}}{{R_{B}\left( {{2Z_{M}} + R_{S}} \right)} + {Z_{M}R_{S}}}} & (11)\end{matrix}$

In order to take a numerical example, it may for example be assumed thatthe light source is a laser with V_(S)=1.6 V and R_(S)=30 ohm.Furthermore, it may for example be assumed that V_(B)=+5 V. If, forexample, a bias-current of 8 mA is desired, then the following isobtained with the help of (10).

-   R_(B)=197.5 ohm

If it is assumed that the modulation current should be 1 mA at 1 Vdifference between V₁ and V₂, and if it is assumed that the capacitorscan be seen as short-circuited at the modulation frequency, then R_(M)is obtained to the following with the help of (11).

-   R_(M)=450.8 ohm

It remains to determine R_(T) such that the total impedance matches thebalanced input impedance of the conductor pair. Without R_(T) it is thecase at higher frequencies (Z_(M)=R_(M)), that the input impedance isthe following.

$\begin{matrix}{R_{IN} = {{2R_{M}} + \frac{2R_{B}R_{S}}{{2R_{B}} + R_{S}}}} & (12)\end{matrix}$

If the obtained numerical values are inserted, then the following isobtained.

-   R_(IN)=929.6 ohm

If, for example, a total input impedance of 100 ohm is desired, thenR_(T) gets the value 112.1 ohm.

From the above described example, it is clear that the transmittercircuit works as it is intended to work and that the circuit can bedimensioned in a simple manner.

FIG. 9 shows another embodiment of the transmitter circuit. Thetransmitter circuit according to FIG. 9 differs from the transmittercircuit according to FIG. 7 in that the third circuit branch 23comprises a fifth resistance R_(T1) and a sixth resistance R_(T2). Theseresistances have essentially the same value. Furthermore, the thirdcircuit branch 23 is arranged with a third constant voltage V_(C)between said fifth R_(T1) and sixth R_(T2) resistances. Furthermore, thetransmitter circuit comprises a transient protection 27 arranged toprotect the light source 20 against undesired voltage pulses.Furthermore, the first circuit branch 21 of the transmitter circuitcomprises a third capacitor C₃. The second circuit branch 22 comprises afourth capacitor C₄. In order to achieve a suitable symmetry, suitablythe third capacitor C₃ has the same value as the fourth capacitor C₄.

The transient protection 27 may be realised in different manners knownto the person skilled in the art. For example, diodes or zener diodesmay be used in order to limit the voltage if it ends up outside acertain interval. With the help of the third capacitor C₃ and the fourthcapacitor C₄, the signal has been AC-coupled before it reaches thetransient protection 27. With the help of the third constant voltageV_(C) and the fifth R_(T1) and sixth R_(T2) resistances, it is securedthat the input signal is around the third constant voltage V_(C) whichis adjusted to the transient protection 27. It is thereby achieved thatthe transient protection 27 only limits the voltage if non-normalvoltages occur. Through the third constant voltage V_(C) and the fifthR_(T1) and sixth R_(T2) resistances, also reflections and other problemsare reduced, since a so-called common-mode termination is achieved whichmeans that signals which are common to the two conductors areterminated.

FIG. 10 shows the principle of the transmitter circuit. As is indicatedwith hatched lines in FIG. 10, the transmitter circuit may comprisefurther cross-connections between the first circuit branch 21 and thesecond circuit branch 22. It is even possible that the transmittercircuit comprises active components. However, preferably passivecomponents are used. Concerning the components which are important forthe normal operation of the transmitter circuit, it is preferably thecase that these components are arranged such that the transmittercircuit is formed mirror-symmetrical along a symmetry line 36 whichpasses through the middle of possible cross-connections. Thereby, theabove described advantages are achieved in a simple manner. Certainparticular components, such as transient protection, which do not haveany influence on the normal operation, do not necessarily have to bearranged with the mirrored symmetry. It should also be noted that thetransmitter circuit may comprise further components. For example, thetransmitter circuit may be arranged with a low-pass filter forpreventing high frequency signals from reaching the light source.

The transmitter circuit has several advantages, such as has already beendescribed above. The input signal does thus not have to be convertedinto an unbalanced signal. This means i.a. that the voltages at thepoints 13 and 15 will be in opposite phases, which means thatdisturbances which could reach other components will be small, sincesuch disturbances from the points 13 and 15 tend to cancel each other.

Now embodiments of an amplifier circuit which may form part of thereceiver unit RXA, RXB of the invention will be described.

FIG. 11 shows an embodiment of the amplifier circuit. According to theshown example, an optical signal is present on a conductor 200. Thesignal is conducted to an input 202 of an amplifier unit 201. Suitablythe optical signal on the conductor 200 is converted to an electricsignal. This is not explicitly shown in the figure. This conversion maybe thought to take place in an input stage which forms part of theamplifier unit 201. The electric signal may either be in the form of acurrent or in the form of a voltage. The amplifier unit 201 is arrangedto influence the amplification of the input signal and to transmit anoutput signal via an output 203. The amplifier unit 201 is according toa preferred embodiment suitably arranged such that a balanced outputsignal is present at the output 203. There may of course also be a unitwhich converts an electric output signal from the amplifier unit 201 toan optical signal before this signal for example is transmitted in anoptical conductor.

It should be noted that by “amplification” is in this document alsocomprised the possibility that the signal is made weaker (amplificationless than 1). This may be the case if the signal is in the form of avoltage. However, preferably a real amplification of the signal takesplace.

It should also be noted that it is not always necessary that the opticalsignal is converted to an electric signal before the signal isamplified. The amplifier unit 201 may thus have both an optical inputsignal and an optical output signal. In this case there may suitably bea transducer 271 which converts the optical output signal to an electricsignal before the signal is conducted to the control units 210, 220, 230described below.

The output signal from the output 203 is conducted to a first controlunit 210 which is arranged to sense said output signal and to deliver afirst control signal at an output 219. This first control signal isintended to control the amplification of the amplifier unit 201. Theoutput signal from the output 203 is also conducted to a second controlunit 220 which also is arranged to sense said output signal and todeliver a second control signal via an output 229. Also the secondcontrol signal is intended to control the amplification of the amplifierunit 201. The first control unit 210 is suitably specially arranged tosense a first kind of signal and the second control unit 220 isspecially arranged to sense a second kind of signal. For example, thefirst control unit 210 may be arranged to sense an output signal whichis continuous or which comprises pulses with a relatively short pausebetween the pulses and the second control unit 220 may be arranged tosense an output signal which comprises pulses with relatively longpauses between the pulses.

As has been described above, the input signals on the conductor 200 canbe optical signals. Such signals may be transmitted as square pulses ofa certain frequency. For example, signals which transfer information mayhave a frequency of about 100 Mbit/s or 1 Gbit/s. Pulses may also arriveas blocks with a frequency of about 10 Mbit/s. When no information istransferred, often so-called link-pulses (or “idle-pulses”) aretransmitted. These pulses may for example have a frequency of about 100Hz, thus an essentially lower frequency than the information carryingsignals. For example, the first control unit 210 may thus be adapted tosense signals with the frequency 10 Mbit/s and faster, while the secondcontrol unit 220 is adapted to sense pulses with a frequency of 100 Hz.

The output signals from the outputs 219 and 229 are conducted to aselector unit 240 which has an output 245 from which a signal isconducted back to an input 204 of the amplifier unit 201 for controllingthe amplification. The selector unit 240 is arranged to control theamplification of the amplifier unit 201 in accordance with that one ofsaid first and second control signals which gives the lowestamplification.

The amplifier circuit may comprise an arbitrary number of control unitsadapted to the different kinds of signals which are the case. In FIG. 11it is indicated with a hatched line a third control unit 230 arranged tovia an output 239 deliver a third control signal to the selector unit240 (of course, the amplifier circuit may comprise more than threecontrol units). The selector unit 240 is arranged to control theamplification of the amplifier unit 201 in accordance with that one ofsaid first, second and third control signals which gives the lowestamplification. The third control unit 230 is suitably specially arrangedto sense a third kind of output signal which differs from the first andthe second kinds of output signals.

FIG. 12 shows an embodiment with three control units where each controlunit comprises a level detector 211, 221, 231 which detects which levelsaid output signal has and an integrator unit 212, 222, 232 whichintegrates the difference between the detected level and a predetermineddesired value 214, 224, 234. The outputs 219, 229, 239 from theintegrator units 212, 222, 232 are connected to the selector unit 240.Also in this case, the different control units are suitably arranged tosense different kinds of output signals. For example, one level detectormay be adapted to sense signals with a relatively long pause between thepulses. Such a level detector may be arranged with a memory such thatthe level of a detected pulse is maintained as output signal from thelevel detector during a certain time.

The level of the signals may be defined in different manners dependingon for which kind of signals the amplifier circuit is used. For example,the level may be an average value of the amplitude of the signal duringa certain time interval. Alternatively, the level may be the maximumamplitude of the signal. According to a preferred embodiment, the leveldetectors may sense the peak-to-peak value of the signals.

As is symbolised with a + and − sign in FIG. 12, the integrator 212,222, 232 integrates the difference between detected level and thedesired value 214, 224, 234. The output signal of the integrator 212,222, 232 thus increases as long as the detected level is higher than thedesired value 214, 224, 234 and decreases if the detected level is lowerthan the desired value 214, 224, 234. For example, the amplifier unit201 may be arranged such that a higher input signal on the control input204 means a lower amplification and vice versa. The selector unit 240selects the control signal which gives the lowest amplification, whichaccording to this example means the highest control signal. According toa preferred embodiment, the selector unit 240 may comprise a pluralityof diode units 241, 242, 243 connected in parallel. A diode unit mayconsist of a diode or of another unit which implements a diode function,for example a transistor connected such that the base corresponds to theanode of a diode and the emitter corresponds to the cathode of thediode. Each diode unit 241, 242, 243 has an input side arranged toreceive a control signal from one of the control units 210. 220. 230, inthis case thus from the integrators 212, 222, 232. The outputs from thediode units 241, 242, 243 are connected to a common point 244. Thesignal from this point is conducted to the input 204. Such a simpleconstruction of the selector unit 240 functions if the input 204 isresistive such that always at least one of the diode Units 241, 242, 243has a forward voltage.

If one of the control units is arranged to sense signals with longerpauses between the pulses, for example the so-called link-pulses, it maysometimes be desirable that this control unit does not control theamplification during normal operation conditions. This may be achievedif the desired value for the integrator unit of this control unit ishigher than the desired value for the integrator unit of the othercontrol unit or control units. Through this higher desired value it isalso avoided that a too low amplification is the case when only thesignal with longer pauses between the pulses controls the amplification,which for example is the case when no other signals are present.

FIG. 13 shows an embodiment which comprises different kinds of controlunits. A first control unit comprises a level detector 211 and anintegrator unit 212 as has been described above. The amplifier circuitcomprises a second control unit which comprises a comparator unit 215,which senses if the output signal from the amplifier unit 201 exceeds apredetermined level, and a signal control unit 216 which has an outputsignal which, at least if a certain condition is fulfilled, changes in afirst direction when the comparator unit 215 senses an output signalexceeding the predetermined level. The output signal from the signalcontrol unit 216 is connected to the selector unit 240. Said firstdirection means a decreased amplification of the amplifier unit 201 ifthe signal from the signal control unit 216 is conducted to theamplifier unit 201. In order to give a concrete example, it may beassumed that the amplifier unit 201 is such that a higher level of thesignal to the input 204 means a lower amplification. Furthermore, theabove mentioned condition may be that the signal should have a frequencywhich exceeds a certain value, for example 10 Hz. This example could forexample be used for sensing the above mentioned link-pulses. Thecomparator unit may for example sense if the amplitude of the pulsesexceeds 1.2V. This means that if link-pulses with an amplitude over 1.2Vare sensed and if these pulses arrive with a higher frequency than 10Hz, then the output signal from the signal control unit 216 isincreased. This means a lower amplification. As mentioned, the selectorunit 240 always selects the amplification in accordance with the outputsignal from the control unit which gives the lowest amplification.

The signal control unit 216 may for example be implemented as a unitwhich has an output signal which increases as soon as the comparatorunit 215 receives a pulse exceeding the predetermined level, butcontinuously or discretely decreases (has a negative ramp) if no suchsignals are received. This means, according to this example, that thesignal control unit 216 delivers a control signal which corresponds toan increased amplification as long as the comparator unit 215 does notreceive any pulses above the predetermined level. For example, it may beassumed that the negative ramp is 100 mV/s and that the increased levelis 10 mV when a pulse exceeding the predetermined level is received bythe comparator unit 215. This thus means that if such pulses arereceived with a frequency which is higher than 10 Hz, then the signalcontrol unit 216 delivers a control signal which corresponds to areduced amplification. If however such pulses are received with afrequency which is lower than 10 Hz, then the signal control unit 216delivers a control signal which corresponds to an increasedamplification.

The control units 210, 220, 230 or parts of these control units and/orthe selector unit 240 may also be implemented with the help of aprogrammable processor unit 260. FIG. 14 shows schematically an exampleof an amplifier circuit which comprises an A/D (analogue-digital)converter 250 which converts the output signal from the amplifier unit201 to a digital signal. Furthermore, by 260 a processor unit issymbolised which treats this digital signal. The processor unit isarranged to form at least a part of the control units 210. 220, 230and/or the selector unit 240. In the shown case, the whole control units210, 220 and the selector unit 240 are formed by a processor unit 260.The amplifier circuit also comprises a D/A (digital-analogue) converterwhich converts an output signal from the processor unit 260 to ananalogue form before this signal is conducted to the amplifier unit 201.This D/A converter is not necessary if the amplifier unit 201 may becontrolled by a digital signal. It also conceivable that a certainelectronic circuitry is present between the output 203 from theamplifier unit 201 and the A/D converter 250. For example, A/Dconversion could also take place after possible level detectors. Thewhole control units do therefore not have to be implemented in theprocessor unit 260.

It should be noted that the amplifier circuit according to a preferredembodiment is arranged such that the amplification is limited such thatit never exceeds a predetermined maximum level. This may for example beachieved in that the amplifier circuit is arranged such that a signalwhich corresponds to a maximum amplification is present at an input,suitably the input 204, of the amplifier unit 201, even if the outputsignal from the selector unit 240 corresponds to a higher amplification.According to an embodiment where a lower signal from the selector unit240 means a higher amplification, this may for example be achieved inthat the amplifier circuit is arranged such that always a minimumcurrent (which corresponds to a maximum amplification) is conducted tothe input 204. Alternatively, it is possible that the control units 210,220, 230 are arranged such that the output signals from these controlunits are limited such that the amplification never exceeds apredetermined value.

Now embodiment of an optical input stage (below called “circuit”) whichmay form part of the receiver unit RXA, RXB of the invention will bedescribed.

FIG. 15 shows such a circuit. The circuit comprises a light sensitivemember 301, for example a photo-diode. In the shown example, the cathodeof the photo-diode is connected to a bias voltage V1. The anode of thephoto-diode 301 is connected to a filter unit 310, in this case acapacitor, which in its turn is connected to a first input 304 of anamplifier component 302. The capacitor 310 prevents a direct currentfrom the photo-diode 301 from reaching the first input 304. The hatchedline. 312 symbolises that such a direct current is conducted away fromthe photo-diode 301. The amplifier component 302 has a first output 306where an amplified output signal is delivered. The amplifier component302 is of the kind which does not have any special input intended forcontrolling the amplification of the amplifier component 302. Such anamplifier component 302 may suitably be of the kind which has beendescribed initially above. Such a component 302 may comprise an internalamplification controlling unit 308. Such an amplification controllingunit 308 may, but does not have to, comprise a feedback control loop.

A control unit 314 is connected to the first input. According to theshown example, the control unit 314 constitutes a variable currentgenerator. With this current generator 314 the current into the firstinput 304 may be controlled. The current generator 314 may thus be usedfor influencing the amplification of the circuit.

It should be noted that the figures only show preferred embodiments. Itis of course also possible that for example the polarity of the circuitmay be the opposite. With reference to FIG. 15, for example thephoto-diode 301 could be reversed and V1 could be a negative voltage.The control unit 314 would in this case control a current out from thefirst input 304.

FIG. 16 shows a further embodiment of the circuit. The correspondingparts as in FIG. 15 have the same reference signs as in FIG. 15. Theseparts will therefore not be described more closely in connection withFIG. 16. According to FIG. 16, the control unit constitutes a variableamplification-controlling voltage unit 316 connected to a firstresistance 318 which in its turn is connected to the first input 304. Asecond diode unit 320 is arranged for preventing an incorrect currentdirection, i.e. in this case a current out from the first input 304. Afilter member 322, in this case a capacitor, is arranged to filter outpossible disturbances which are superposed on theamplification-controlling voltage.

A second resistance 324 is connected to the anode of the photo-diode 301for conducting away a direct current. According to the shown embodiment,the amplifier component 302 is of the kind where a low current into thefirst input 304 means a high amplification. In order to ensure that acertain current is always present at the first input 304, a thirdresistance 326 is arranged between the bias voltage V1 and the firstinput 304. This third resistance 326 thus limits the amplification ofthe circuit.

It may be interesting to measure the photo current through thephoto-diode 301. The photo current is proportional to the voltage overthe second resistance 324. However, it may be unsuitable to measure thisvoltage since such a measurement could lead to disturbances in asensitive part of the circuit. In order to avoid this problem, a fourthresistance 330 is connected to the anode of the photo-diode 301. Thisfourth resistance is suitably a resistance with a high resistance. Ameasurement device 332 may thus be connected to this fourth resistance330.

334 symbolises a circuit which follows after the amplifier component302. This circuit 334 may comprise an amplifier. The hatched line 336symbolises that a feedback from this circuit 334 may be arranged forinfluencing the variable voltage which is symbolised by the unit 316.

It will now briefly be described how the different parts of theinvention which have been described above are arranged in thetransmitter-receiver device (A, B) which is shown in FIG. 4.

As has been mentioned above, the above described transmitter circuit mayform part of the transmitter unit TXA (or TXB). With reference to forexample FIG. 7, it may thus be noted that the twisted pair 30 ofconductors corresponds to the line 133 in FIG. 4. The fibre 35 in FIG. 7corresponds to the fibre F2 in FIG. 4. The signal in at the input 103 inFIG. 4 may correspond to the voltage V_(B) in FIG. 7. The voltage V_(B)may thus be switched on or switched off with the help of the supervisingunit CUA in FIG. 4. Thereby also the optical power which is transmittedon the fibre F2 may be switched on and switched off. The sixth output115 in FIG. 4 does not have any direct correspondence in FIG. 7. Thesignal at the output 115 may for example be controlled by an arbitrarylevel detector which detects if a balanced input signal is present atthe pair 30 of conductors. A sufficiently strong balanced signal meansthat the transmitter unit TXA receives an information carrying signal atthe input 143.

With reference to FIGS. 11 to 14 it may be noted that the opticalconductor 200 corresponds to the fibre F1 in FIG. 4. The line 131 inFIG. 4 corresponds to the line from the output 203 in FIGS. 11 to 14.

As has been mentioned above, the circuit according to FIGS. 15 and 16may form part as an input stage in the amplifier unit 201 in FIGS. 11 to14. With reference to for example FIG. 16, it may thus be noted that thelight which falls on the photo-diode 301 corresponds to the light fromthe fibre F1 in FIG. 4. The input 204 in FIGS. 11 to 14 may be said tocorrespond to an input which controls the current generator 314 in FIG.15 or to for example the input to the diode unit 320 in FIG. 16. Thesignal which controls the amplification in at the input 204 thuscorresponds to a signal to the current generator 314 in FIG. 15,alternatively a signal at the line to the diode unit 320 in FIG. 16,which signal is conducted to the input 304. The signal from the output101 in FIG. 4, which signal indicates that the receiver unit RXAreceives light, may correspond to a signal from the measurement device332 in FIG. 16. The measurement device 332 may for example be acomparator which detects if the voltage over the resistance 330 is abovea predetermined value. The circuit 334 in FIG. 16 may be thought tocomprise an amplifier unit, which may form part of the amplifier unit201 according to FIGS. 11 to 14. Also a part of the feedback loop whichis shown in FIGS. 11 to 14 may be thought to form part of the circuit334 in FIG. 16. The diode unit 320 in FIG. 16 may for example constitutediode units, connected in parallel, which form part of the selector unit240 according to FIGS. 11 to 14. Of course, the diode unit 320 may alsobe a separate diode unit.

It should be noted that the output 111 in FIG. 4, at which output asignal is present which indicates whether the receiver unit RXA receivesan information carrying signal via the first optical fibre F1, does nothave any direct correspondence in the other figures. This signal may beobtained in different manners. For example, the level of the signal fromthe selector unit 240 in FIGS. 11 to 14 may be detected. For example, ifa low level at this signal means a high amplification, the following maybe the case: if the level is below a predetermined value, whichcorresponds to a high amplification, then it is assumed that noinformation carrying signal, i.e. no modulated signal, is received atthe input 202, since if such an information carrying modulated signalwhere the case, the amplification determined by the selector unit 240would not be so high.

It should be noted that when in this document signals are mentioned,these signals may be balanced, i.e. differential, even if they are notalways described in this way. Particularly suitable is that theinformation carrying signals at the lines 131 and 133 in FIG. 4 aredifferential.

The invention is not limited to the shown embodiments but may be variedwithin the scope of the annexed claims.

1. A transmitter-receiver device comprising a receiver unit arranged tovia a first optical conduction path receive light and optical signalsand comprising a first output which indicates whether the receiver unitreceives light, a transmitter unit arranged to on a second opticalconduction path transmit light and optical signals and comprising afirst input which controls whether the transmitter unit shall transmitlight, a supervising unit with a second input connected to said firstoutput and a second output connected to said first input and arranged tovia said second output prevent the transmitter unit from continuouslytransmitting light when the supervising unit via the second inputdetects that the receiver unit does not receive light, wherein thesupervising unit is arranged to, when it detects that the receiver unitdoes not receive light, change to a test mode where the supervising unitcontrols the transmitter unit to intermittently transmit short lightpulses on said second optical conduction path, wherein the supervisingunit is arranged with a third out-put where a status signal indicateswhether the transmitter-receiver device is in said test mode, whereinthe transmitter unit comprises a transmitter circuit comprising a lightsource and arranged to operate said light source to transmit opticalcommunication signals in response to electric input signals from a firstand a second circuit point between which circuit points a balancedelectric input signal is intended to be present, wherein saidtransmitter circuit comprises a first circuit branch which extends fromsaid first point via a third point to at least a fourth point andwherein said transmitter circuit comprises a second circuit branch whichextends from said second point via a fifth point to at least a sixthpoint, wherein said light source is connected between said third andfifth points, wherein the components which are positioned on said firstand second circuit branches are chosen such that the transmitter circuitis formed with a symmetry which is such that under normal operationconditions a balanced drive voltage is the case between said third andfifth points, which balanced drive voltage only depends on the voltagedifference between said first and second points, wherein also themodulation current through the light source only depends on said voltagedifference.
 2. A transmitter-receiver device according to claim 1,wherein the supervising unit is arranged such that when thetransmitter-receiver device is in said test mode, the time between saidlight pulses is less than 1 s, preferably less than 0.1 s.
 3. Atransmitter-receiver device according to claim 1, wherein the receiverunit is arranged with a fourth output which is connected to thesupervising unit, at which fourth output a signal is the case whichindicates whether the receiver unit, via the first optical conductionpath, receives an information carrying input signal, wherein thesupervising unit has a fifth output with a status signal which dependsboth on the status of the signal of said third output and the status ofthe signal from said fourth output.
 4. A transmitter-receiver deviceaccording to claim 1, wherein the transmitter unit is arranged with asixth output which is connected to the supervising unit at which sixthoutput a signal is the case which indicates whether the transmitter unitreceives an electric information carrying input signal, wherein thesupervising unit has a seventh output with a status signal which dependsboth on the status of the signal of said third output and the status ofthe signal from said sixth output.
 5. A transmitter-receiver deviceaccording to claim 1, wherein said first and second circuit branches areformed with a mirrored symmetry, such that the electric properties ofthe components which are arranged on said first circuit branchcorrespond to the same electric properties of the components which arearranged on said second circuit branch.
 6. A transmitter-receiver deviceaccording to claim 1, arranged such that a first constant voltage is thecase at said fourth point and a second constant voltage is the case atsaid sixth point.
 7. A transmitter-receiver device according to claim 1,wherein all the components which are arranged on said first and secondcircuit branches are passive components.
 8. A transmitter-receiverdevice according to claim 1, wherein the receiver unit comprises anamplifier circuit comprising at least: an amplifier unit arranged toreceive an input signal and to influence the amplification of this inputsignal and to transmit an output signal which is intended to lie at adesired level, a first control unit arranged to detect said outputsignal and to deliver a first control signal for controlling theamplification of said amplifier unit, a second control unit arranged tosense said output signal and to deliver a second control signal forcontrolling the amplification of said amplifier unit, a selector unitarranged to receive said first and second control signals and connectedto said amplifier unit, wherein the selector unit is arranged to controlthe amplification of the amplifier unit in accordance with that one ofsaid first and second control signals which gives the lowestamplification.
 9. A transmitter-receiver device according to claim 8,wherein the first control unit is specially arranged to sense a firstkind of output signal and wherein the second control unit is speciallyarranged to sense a second kind of output signal.
 10. Atransmitter-receiver device according to claim 9, wherein the firstcontrol unit is specially arranged to sense an output signal which iscontinuous or which comprises pulses with a relatively short pausebetween the pulses and wherein the second control unit is speciallyarranged to sense an output signal which comprises pulses withrelatively long pauses between the pulses.
 11. A transmitter-receiverdevice according to claim 1, wherein the receiver unit comprises acircuit for receiving an optical signal, which circuit comprises: alight sensitive member arranged to receive an optical input signal andto deliver an electric signal in response to the received opticalsignal, an amplifier component with a first input arranged to receivethe electric signal from the light sensitive member, wherein theamplifier component is arranged to influence the amplification of theelectric signal and to deliver an amplified output signal via a firstoutput, wherein the amplifier component does not have any further inputspecially intended for controlling the amplification of the amplifiercomponent, a filter unit arranged to prevent a possible direct currentin the electric signal from the light sensitive member from reachingsaid first input, and a control unit connected to said first input andarranged to control the power of the electric signal at this first inputfor thereby influencing the power of said output signal.
 12. Atransmitter-receiver device according to claim 11, wherein said lightsensitive member is arranged such that a possible direct current fromthe light sensitive member would be directed into said amplifiercomponent via the first input if said filter unit were not arranged toprevent such a direct current, wherein the control unit is arranged tocontrol the power of an electric current in a direction in via the firstinput.
 13. A transmitter-receiver device according to claim 11, whereinsaid light sensitive member comprises a first diode unit with a firstconnection connected to a bias voltage and a second connection connectedto said filter unit, which filter unit is connected to the first input.14. A transmitter-receiver device according to claim 11, comprising anamplification limiting unit connected to said first input.
 15. Atransmitter-receiver device according to claim 11, comprising a seconddiode unit connected to said control unit for preventing incorrectcurrent direction to/from the first input.
 16. A communication systemcomprising a first transmitter-receiver device according to claim 1, anda second transmitter-receiver device according to to any of thepreceding claims, and a first and a second optical conduction path whichconnect the first and the second transmitter-receiver device to eachother, wherein the first optical conduction path is connected to thereceiver unit of the first transmitter-receiver device and thetransmitter unit of the second transmitter-receiver device, wherein thesecond optical conduction path is connected to the receiver unit of thesecond transmitter-receiver device and the transmitter unit of the firsttransmitter-receiver device.
 17. A communication system according toclaim 16, comprising a network management system, wherein at least oneof said first and second transmitter-receiver devices is connected tothe network management system.
 18. A communication system according toclaim 17, wherein at least said third output of said at least onetransmitter-receiver device is connected to the network managementsystem.
 19. A communication system according to claim 18, wherein atleast said at least one transmitter-receiver device is arranged suchthat also said fifth and seventh outputs are connected to the networkmanagement system.
 20. A communication system according to claim 16,arranged such that said third, fifth, and seventh outputs of the firsttransmitter-receiver device, except for possibly during a short timedelay, have the same status as the third, seventh, and fifth,respectively, outputs of the second transmitter-receiver device.